For a communication apparatus, such as a cellular phone, reductions not only in size and weight but also in cost are keenly called for. This cost saving can be effectively accomplished by reducing such external parts as a voltage control oscillator (VCO) and a SAW filter. According to the Global System for Mobile Communications (GSM), which is one of the digital mobile phone systems currently used in Europe, the main stream is an offset PLL formula, which does not use the expensive SAW filter. On the other hand, there is a direct conversion formula according to which high frequency oscillation signals, which constitute the carrier, are directly modulated with transmission data. This direct conversion formula would permit a further saving in cost because the IF-VCO, which generates oscillation signals of an intermediate frequency, can be eliminated.
In recent years, cellular phones have come to be required to have a dual band formula capable of handling signals of two frequency bands, such as the GSM of the 880 to 915 MHz band and the digital cellular system (DCS) of the 1710 to 1785 MHz band, or a triple band formula capable of handling signals of the personal communication system (PCS) of the 1850 to 1910 MHz band in addition to the GSM and the DCS. Conceivably, cellular phones will be required to be adaptable to even more bands in the future.
In a semiconductor integrated circuit (hereinafter referred to as a modulating/demodulating LSI) for modulating transmission signals and reception signals for use in such a cellular phone adaptable to a plurality of bands, providing a modulating circuit and a demodulating circuit for handling signals of each of the multiple bands would entail the problem of an increased circuit area and accordingly a higher chip cost. In view of this problem, it is conceivable to have the plurality of bands to use a modulating circuit and a demodulating circuit in common. However, an attempt at common use of a modulating circuit and a demodulating circuit would encounter difficulty to satisfy the requirements regarding the output level and noise characteristics.
More specifically, in the transmission circuitry for instance, if a conventional orthogonal modulating circuit having a resistor as the load element is used as it is, it will be difficult to keep the level of noise leaking into the reception frequency band (C/N ratio) as low as required. In the reception circuitry on the other hand, common use of mixers at a later stage by multiple bands would allow the output signal of the low noise amplifier (LNA) of the selected band to leak into the output of the LNAs of unselected bands, resulting in a drop in the output level of the LNA of the selected band and accordingly a deterioration in noise figure (NF) when amplification is performed by the mixer at a subsequent stage.
An object of the present invention is to provide a semiconductor integrated circuit for communication equipped with a modulating circuit capable of satisfying the noise characteristic requirement (C/N ratio) when an orthogonal modulating circuit is used in common by a plurality of bands to be adaptable to those multiple bands in the transmission circuitry of a direct up conversion formula with a view to restraining an increase in chip area.
Another object of the invention is to provide a semiconductor integrated circuit for communication equipped with a demodulating circuit capable of avoiding a deterioration in noise figure (NF) when a mixer for demodulating reception signals is used in common by a plurality of bands to be adaptable to those multiple bands in the reception circuitry of a direct down conversion formula with a view to restraining an increase in chip area.
Still another object of the invention is to provide a semiconductor integrated circuit for communication capable of communication over a plurality of bands and, moreover, requiring neither a SAW filter nor an IF-VCO for generating oscillation signals of an intermediate frequency, both of which would be external parts, and thereby enabling the number of constituent parts required to be reduced.
The above-stated and other objects and novel features of the invention will become more apparent from the following description in the specification when taken in conjunction with the accompanying drawings.